1. Field of the Invention
The present invention relates to technique of generating electromagnetic radiation in response to received light, and more specifically, to technique of generating electromagnetic radiation from a plane surface.
2. Description of the Background Art
Researches have been made in recent years on technique what is called imaging that forms an image of physical information about a test target by using electromagnetic radiation in a terahertz region (hereinafter also called terahertz wave). A terahertz wave has the properties of both a radio wave and light, and is expected to be applied in industrial purposes for its high performance in passing through an object and in consideration of safety (non-patent literature 1). In the field of security, body scanners already put into practical use and placed for example in air ports use a terahertz wave (mainly a sub-terahertz wave) as a way to pass through an object safely.
Generally, for imaging using a terahertz wave, a test target is caused to move two dimensionally relative to a terahertz wave emitted from an electromagnetic radiation generating element, or the test target is scanned two dimensionally with electromagnetic radiation by using optical technique. In such irradiation with electromagnetic radiation, the test target is scanned relatively with electromagnetic radiation, leading to a complicated device structure. So, technique of generating terahertz waves from a two-dimensional plane surface has already been suggested (non-patent literature 2).
Non-patent literature 2 describes generation of terahertz waves by irradiating DAST crystal being nonlinear optical crystal with femtosecond laser. The DAST crystal is scanned with pulsed light by using a galvanometer mirror, thereby generating terahertz waves from a two-dimensional plane surface.
Non-patent literature 1: “Current status and future prospects of terahertz technology” from Applied Physics, p. 160, Vol. 75, No. 2 (2006), written by Masayoshi Tonouchi
Non-patent literature 2: Technical report of the Institute of Electronics, Information, and Communication Engineers, pp. 9-13, vol. 110, No. 66, LQE 2010-3, May, 2010 written by Serita, Tonouchi et al.
However, DAST crystal is a very costly material if it is used as an electromagnetic radiation generating element. Further, large-sized DAST crystal of a size of some millimeters or larger is hard to obtain easily, if manufacture of such DAST crystal is not technically impossible. So, it is in fact difficult to generate electromagnetic radiation from an extensive plane surface by using DAST crystal.